The suicides of two teenage girls who fell more than 100ft from a bridge may have been avoided if more staff had been on duty at the care unit they were living in, a sheriff has said.

Niamh Lafferty, 15, and Georgia Rowe, 14, fell over the edge of the Erskine Bridge into the River Clyde below in October 2009. Both girls, who were in care at the time, had been residents of the nearby Good Shepherd Centre in Bishopton, Renfrewshire.

A fatal accident inquiry into their deaths was heard over 65 days last year at Paisley Sheriff Court. A written determination in the case by Sheriff Ruth Anderson has now been published.

"There was no accident. Both deaths were suicides," she said, adding that the deaths may have been avoided "had the following reasonable precautions been taken".

These two precautions, she said, were having at least four members of staff on duty at the centre's open unit on October 4 2009, and placing the two girls on the first floor rather than in the self-contained flat on the ground floor "directly opposite an unalarmed fire exit door". She added: "There were no defects in the system of working which contributed to the deaths."

The sheriff said both girls died as a result of injuries due to a fall from height. In her written determination, she said the deaths might have been avoided "on that day" if the two "reasonable precautions" had been taken on October 4. She said: "I had no hesitation, in light of all the evidence, in concluding that Niamh and Georgia were well aware of what they were doing, and the consequences for them.

Evidence from social workers and family was heard throughout the inquiry about how the girls came to be in care. The inquiry heard that Niamh, from Helensburgh, Argyll, was disturbed by the loss of her 16-year-old boyfriend eight months before and she had tried to take her own life in the months that followed.

Concerns were raised about 14-year-old Georgia's mental health five years before she died, when she told social workers she was going to kill herself, the inquiry was told. But the inquiry heard that despite all their troubles, both girls were said to have spent a happy weekend with relatives before they died.

Sheriff Anderson said: "I would wish once more to express my sympathy to the families of Niamh and Georgia and to put on record how grateful I am for the way in which those who attended the inquiry conducted themselves throughout, both in the giving of evidence and in the way they conducted themselves during the many days of evidence, much of which must have been distressing for them all.

The Good Shepherd Centre open unit is now closed but a statement was released on behalf of the board of managers of the centre's secure unit expressing "renewed condolences" to the girls' families. The board said it was grateful to the sheriff for "careful and patient consideration of the evidence" and for the extensive findings. It said that while the open unit no longer exists, "any lessons which may be learned will be implemented in full, where they may be applicable to the secure unit, and this will undoubtedly be true also for other open and secure units across Scotland dealing with vulnerable young people".